HOME FIRE PROTECTION

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A SMOKE DETECTOR CAN PROVIDE VITAL EARLY WARNING TO PROTECT YOU AND YOUR FAMILY ...the perils of a home conflagration

by David Schoonmaker

EACH YEAR, BETWEEN 5,000 and 6,000 people in the U S. die as a result of structural fires, ranking it fourth as a cause of accidental death. Only car accidents, falls, and drowning claim more lives. Of these fatalities, more than 80% occur in one- and two-family homes.

Happy, there are significant fewer fire deaths today than there were 10 years ago—this despite more than a doubling in property losses owing to fire during the same period. There's a simple explanation for the declining number of deaths in the face of increasing dollar losses. It's called the smoke detector.

The Hardware

There are two basic types of smoke detectors. An ionization unit contains a small amount (less than one microcurie) of a radio-active substance, such as americium-241. The decaying material fills one or two sampling chambers with ions of nitrogen and oxygen, which support a very small electrical current that keeps the alarm silent. Should smoke enter the chamber(s), however, the ions attach to the particles, and the current path breaks down, sounding the alarm.

Photoelectric smoke alarms rely on light scattering to detect smoke. As long as light emitted periodically by a diode (LED) fails to strike a photocell set out of line of the light path, the alarm stays off. But if smoke enters the light-tight chamber, the particles will reflect and refract the light onto the photocell, sounding the alarm.

Each type has its advantages. Because it depends on an ion-strewn current path, the ionization detector responds better to smoke consisting of numerous small (less than one micron) particles—typical of the emissions from a blazing fire. It will, for example, respond quickly to toxic gases given off by certain burning plastics. Unfortunately, it's also quite sensitive to the fumes produced in a busy kitchen. A photoelectric detector responds more quickly to the large (greater than one micron) particles produced by smoldering fires. It will warn earlier of the sort of fire caused by a careless smoker or by spontaneous combustion, And it's much less prone to sound nuisance alarms. Its main failing is a comparative insensitivity to fumes and to black particles, which absorb rather than reflect light.